1. Technical Field
The present invention is directed toward communication systems, and more particularly toward pilot symbol assisted wireless systems.
2. Background Art
Signals for wireless systems are subjected to varying conditions which can degrade the signal received by the mobile units using the system.
For example, a mobile unit can receive the signal from multiple directions (e.g., directly from the base unit, and reflected off of many different ground objects), with the varying signal sources potentially being out of phase and thereby tending to cancel each other out to some degree, reducing signal strength. Such signal fading, generally known as Rayleigh fading, occurs spatially over the area of the system, with specific areas potentially having significant fading which could cause the mobile unit to lose the signal entirely.
Still further, mobile units are subject to the Doppler effect as they move about the system. As is known in the art, whenever relative motion exists between the signal source/transmitter and signal recipient/receiver, there is a Doppler shift of the frequency components of the received signal. Thus, where the recipient is in a vehicle moving at a speed v, the maximum Doppler shift frequency fd (occurring when the vehicle is moving directly at or directly away from the signal source) is:
fd=v/xcex
Component waves arriving from ahead of the vehicle experience a positive Doppler shift (i.e., the frequency increases) while those arriving from behind the vehicle have a negative shift (i.e., the frequency decreases). Thus, if a vehicle is traveling 60 km/hr, at 900 MHz (xcexxcx9c0.33 m) the maximum Doppler shift (when the vehicle is traveling directly toward or away from the signal source) is:
fd=[60,000 m/hr/3600 sec/hr]/0.33 m=50 Hz
Of course, a proportional change in frequency, or speed, would produce a proportional change in fd. This shift in frequency results in the maximum signal strength being at the shifted frequency rather than the assigned frequency, with the signal strength being significantly less at the assigned frequency (as perceived by the moving mobile unit) which is demodulated by the mobile unit. If the mobile unit happens to also pass through an area in the system subjected to significant Rayleigh fading, a significant loss in signal strength can accordingly result.
In any event, the net result of these and other factors is that the signal which is transmitted by the transmitter (e.g., a cell tower) will be distorted by the time it reaches the receiver (e.g., cellular telephone). In a cellular telephone, for example, this can result in distortion objectionable to the ear, or even a lost signal.
In order to account for this distortion, channel estimates have been used to determine the signal distortion at known pilot symbols in the data bursts and correction factors at other symbols in the data bursts have been interpolated based on the channel estimates at the pilot symbols. As an example, data bursts have been transmitted in the IS-136 System with 162 symbols, each symbol comprising two bits. In a proposed extension of the IS-136 System, the data bursts of 162 symbols at predetermined, known locations Pi in the data bursts are predetermined, known pilot symbols SPi (where i =1 to n, n being the number of pilot symbols used). In the proposed extension of the IS-136 System, each symbol contains three bits.
As also described further below, the correction factors (i.e., channel estimates) derived from the pilot symbols can be used to estimate the most likely value for each data symbol in a data burst. That is, the channel estimates derived from the pilot symbols may be interpolated to determine the correction factors at the other symbols (i.e., data symbols) in the data burst by using an interpolator or filter selected to best work under the conditions most likely to be encountered by the communication unit. In order to provide acceptable performance, such interpolator or filter essentially needs to be designed so as to handle the highest possible vehicle speed. For example, an interpolator or filter designed to accommodate a Doppler effect encountered at 60 kph will not provide acceptable performance for a vehicle traveling at 70 kph toward or away from the cell tower). Unfortunately, this has unavoidably resulted in required use of an interpolator or filter which causes degradation in the estimation of symbols which are received under conditions other than those parameters upon which the interpolator or filter is designed (e.g., degradation at a low vehicle speed). This degradation can be even worse in areas where very high vehicle speeds must be anticipated since the interpolator must be designed based on very high anticipated speeds which oftentimes will not be encountered.
The present invention is directed toward overcoming one or more of the problems set forth above.
In one aspect of the present invention, a mobile communication unit which is subjected to conditions which degrade the receipt of a signal is provided. The unit includes a receiver adapted to receive a signal having multiple symbols therein including predetermined pilot symbols, a processor adapted to demodulate received symbols based on an interpolator using the error in the received pilot symbols, and an output adapted to receive symbols demodulated using the interpolator which is best adapted to correctly demodulate selected ones of the received symbols under the conditions to which the communication unit is subjected when the symbols being demodulated are received. The processor selects that interpolator from at least two possible interpolators which is best adapted to correctly demodulate the received symbols under the conditions to which the communication unit is subjected when the symbols being demodulated are received.
In a preferred form of this aspect of the present invention, the received signals include error detection coding and an error detection decoder decodes the signal using the at least two interpolators with the processor selecting that interpolator which the decoder detects as having the least errors as the best adapted interpolator.
In another preferred form of this aspect of the present invention, the selected ones of the received symbols comprise less than half of the received symbols and, in one preferred form, the selected ones of the received symbols are from more than one data burst in the signal.
In still another preferred form of this aspect of the present invention, the mobile communication unit also includes memory maintaining at least two interpolators. In one alternate, the processor of this preferred form demodulates the selected ones of the received symbols using all of the at least two interpolators and selects that interpolator which has the least cumulative error in the demodulated symbols from discrete possible values of the symbols as the one best adapted to correctly demodulate the received symbols. In another alternate of this preferred form of this aspect of the present invention, the communication unit includes an estimator for determining the conditions to which the communication unit is subjected when the symbols that need to be demodulated are received, and the memory also stores information regarding the conditions under which each of the at least two interpolators is best adapted to correctly demodulate the received symbols. In a preferred form of this other alternate, the estimator is an algorithm estimating the Doppler shift of the unit, and the information in the memory is the range of Doppler shifts at which each of the at least two interpolators is best adapted to correctly demodulate the received symbols.
In another preferred form of this aspect of the present invention, the mobile communication unit includes memory storing a first algorithm for deriving any of a plurality of interpolators based on selected conditions to which the communication unit is subjected when symbols being demodulated are received, and also includes a sensor for determining the selected conditions to which the communication unit is subjected when the symbols being demodulated are received, with the processor using the selected conditions determined by the sensor to derive an interpolator from the algorithm. In a preferred form, one selected condition determined by the sensor is the Doppler shift and the first algorithm stored by the memory derives interpolators based on Doppler shift. In another preferred form, the sensor is a second algorithm for determining the Doppler shift and the first algorithm stored by the memory derives interpolators based on Doppler shift.
In another aspect of the present invention, a mobile unit is provided for communicating with a transmitter which transmits signals in data bursts, such data bursts having a plurality of symbols therein including data symbols and a plurality of predetermined pilot symbols. The symbols have a discrete number of possible values. The unit further includes a receiver adapted to receive a data burst of the transmitted signals, memory with the predetermined pilot symbols and a plurality of signal filters, a comparator, a processor, and an output. The comparator is adapted to compare the pilot symbols in a received data burst with the predetermined pilot symbols in the memory to determine channel estimates at the locations of the pilot symbols. The processor is adapted to use the pilot symbol channel estimates and the plurality of signal filters to derive a set of data symbol correction factors for each of the signal filters. The processor is adapted to adjust the discrete possible values of the data symbols by the data symbol correction factors and compare the adjusted data symbols with selected received data symbols to determine a cumulative error value among the selected received data symbols for each signal filter. The output is adapted to receive symbols demodulated by the processor using the interpolator having the lowest cumulative error value.
In a preferred form of this aspect of the present invention, the processor is adapted to measure an error as the difference between a selected received data symbol and the closest adjusted discrete possible value of the data symbols, and the cumulative error value for each signal filter is the sum of the squares of the absolute values of the error at each selected received data symbol.
In another preferred form of this aspect of the present invention, the cumulative error value is determined from selected received data symbols from more than one data burst.
In still another aspect of the present invention, a mobile unit is provided for communicating with a transmitter which transmits signals. The unit includes a receiver adapted to receive the transmitted signals, memory with a plurality of signal filters, the signal filters being selected to correct for selected Doppler shifts, an estimator estimating Doppler shift of the unit, and a demodulator responsive to the estimator for demodulating the transmitted signal as received by the receiver using the selected one of the signal filters which best corrects for the estimated Doppler shift.
In a preferred form of this aspect of the present invention, the estimator is an algorithm for estimating the Doppler shift of the unit.
In another preferred form of this aspect of the present invention, the estimator is a processor which for each signal filter compares selected symbols in the received signal with discrete possible values of the symbols adjusted by the each filter to determine an error figure for each signal filter, the estimated Doppler shift falling in a range of Doppler shifts best corrected by the signal filter having the smallest determined error figure.
In yet another aspect of the present invention, a method is provided for improving the signal reception of a mobile communication unit, including the steps of (a) transmitting a signal from a base station with data symbols having discrete possible values and pilot symbols with predetermined values at predetermined locations in the signal, (b) receiving the transmitted signal at the mobile communication unit, (c) deriving correction factors at the predetermined locations of the pilot symbols in the signal by comparing the symbols as received by the mobile communication unit with the predetermined values of the pilot symbols, (d) using a plurality of interpolators to interpolate corrections for the data symbols based on the correction factors from step (c), (e) correcting the data symbols using the interpolated corrections from step (d) and comparing the received data symbols to the adjusted discrete possible values for the data symbols to generate a cumulative error value for each interpolator, (f) selecting a set of data symbols which are the possible values closest to the corrected data symbols from the interpolator with the lowest cumulative error value, and (g) outputting the received signal using the selected data symbols.
In a preferred form of this aspect of the present invention, two interpolators are used in step (d), one interpolator being specially adapted to interpolate corrections for a communication unit traveling at high speeds and the other interpolator being specially adapted to interpolate corrections for a communication unit traveling at low speeds.
It is an object of the invention to provide mobile communication units such as cellular telephones which will provide maximum reliability and highest signal quality.
It is another object of the invention to provide mobile communication units which can demodulate high quality signals using current transmission standards.